Processing of the amyloid precursor protein (APP) leading to the accumulation of the A? fragment, is a critical component of the pathogenesis of Alzheimer's Disease (AD). ApoE is another important molecule in AD pathogenesis, although its functions in the CNS are even less well understood. Recently, we found that Dab1, an adaptor protein that binds the cytoplasmic domains of APP and apoE receptors, affects APP processing and A? levels, and processing of the apoE receptor, ApoEr2. Dab1 also associates with Fyn kinase, and upregulation of Dab1 is found in mice carrying mutations in Fyn. Fyn is a Src family tyrosine kinase with diverse biological functions due to its ability to phosphorylate a wide variety of intracellular signaling molecules in response to extracellular stimuli. Most notably, Fyn is involved in modulating signal transduction processes in neurons involved in learning and memory, perhaps through effects on NMDA receptor subtypes. In preliminary studies for this proposal, we have found that altering Fyn levels or activity causes changes in APP trafficking, APP stability, and increased A? levels. We also found that Fyn had similar effects on ApoEr2. We hypothesize that Fyn phosphorylation of APP decreases its amyloidogenic processing. We will test whether Fyn and Dab1 work in conjunction or compete with one other to alter processing of APP and ApoEr2. Finally, we hypothesize that physiological activation of Fyn promotes these effects on APP and ApoEr2 and is important for their regulation. In Aim1, we will test the mechanism of action of Fyn kinase on APP and ApoEr2 proteolysis. In Aim 1A, we will determine whether phosphorylation of APP, ApoEr2, and Dab1 affect their associations and the processing. In Aim 1B, we will determine whether phosphorylation and processing of APP and ApoEr2 are altered in Fyn knock-out cells, in vitro and in vivo, or after reduction of Fyn with siRNA. In Aim 2, we will test the effects of Fyn activation in neurons. We will initially (Aim 2A) focus on the effects of Fyn on the trafficking of APP and ApoEr2 to the cell surface, and define whether these effects modulate the effect of Fyn on NMDA receptor trafficking. Secondly (Aim 2B), we will use reelin as a physiological activator of Fyn to define how activation of Fyn affects neuronal trafficking and processing of APP and ApoEr2. Through these experiments, we will test how Fyn regulates the trafficking, stability and cleavage of APP, thus affecting A? levels.